1. Field of the Invention
This invention relates to an ultrasonic diagnostic apparatus, and particularly to a contrast medium imaging technique.
2. Description of the Related Art
Microbubbles contained in a contrast medium used for ultrasonic diagnosis are collapsed by ultrasonic transmission and tend be collapsed when the transmitted sound pressure is higher. To maintain a contrast effect, a certain measure needs to be taken such as imaging a contrast medium while suppressing its collapse by using a low sound pressure (real-time perfusion image or RPI).
The case of extracting a second harmonic component of a transmitted fundamental wave from a received echo in RPI will now be considered. When the sound pressure is high, the second harmonic component contains a nonlinear propagation component generated in the tissue.
When the sound pressure is low as in RPI, the nonlinear propagation component generated from the tissue has very low intensity, which is insufficient for imaging. A tissue image can hardly be observed before inflow of a contrast medium, and only after inflow of the contrast medium, a harmonic component due to the contrast medium begins to appear.
In short, in the ultrasonic test using a contrast medium, when the sound pressure is lowered to maintain the contrast effect, a nonlinear propagation component generated in the tissue has very low intensity and therefore a tissue image hardly appears before inflow of the contrast medium. On the other hand, when the sound pressure is raised to enable appearance of the nonlinear propagation component generated from the tissue before inflow of the contrast medium, the contrast effect momentarily disappears. Particularly, this problem is noticeable at the time of ultra-low sound pressure driving where an MI (mechanical index) value, which is an index showing an output reference obtained by normalizing a maximum peak negative sound pressure in a transmitted beam by the square root of the fundamental frequency, is approximately 0.1.
To solve this problem, it may be conceivable to use a conventional color Doppler processing unit to prepare and display a B mode with a fundamental wave as a tissue (background) image and to display a Doppler image (including phase inversion Doppler) as a contrast image. However, since separate transmissions and receptions are necessary for the tissue and for the contrast, respectively, the frame rate, which is particularly important for imaging a cardiovascular system, is lowered and the real-time property cannot be utilized. Moreover, the transmission for the tissue may cause unwanted collapse of and adverse effects on the contrast medium.
When an image is generated using only a harmonic component with a low MI value, even if contrast-enhancement is performed, the brightness of the image is low and it may be difficult to confirm the enhanced region. If the gain is increased only to increase the brightness, noise appears in the image. Even if the dynamic range is narrowed to brighten a maximum brightness part, only a part having relatively high brightness is emphasized and the contrast-enhancement cannot be correctly evaluated.